DC-DC converter and regulation method for this

ABSTRACT

A DC-DC converter, a regulation method for a DC-DC converter and a switched-mode power supply are proposed. The DC-DC converter comprises an inverter and a primary-side circuit with a transformer whose secondary-side voltage is rectified by at least one rectifier for generating an output DC voltage. To avoid an asymmetrical load, which is in particular exhibited by a different load of the rectifier elements (power semiconductors), an electrical magnitude of the DC-DC converter is measured. This magnitude may be, for example, a primary-side current, a primary-side voltage at a capacitance, or a secondary-side, rectified voltage. From the measurement of the magnitude is calculated a parameter for the symmetry deviation for which different symmetry measuring methods are proposed. A symmetry regulation arrangement utilizes the drive of the inverter, for example, the duty cycle of the pulse width modulated voltage produced by the inverter to minimize the parameter for the symmetry deviation. This achieves an even distribution of the power over the secondary rectifier elements.

[0001] The invention relates to a DC-DC converter, a regulation methodfor a DC-DC converter and a switched-mode power supply.

[0002] DC-DC converters, also denoted converters, are known forconverting an input-side DC voltage into an output-side DC voltage. A DCvoltage present on the input side is first converted (chopped) into aswitched AC voltage and this AC voltage feeds a circuit which comprisesthe primary side of a transformer. In this way at least one voltagewhich after rectification is available as an output DC voltage isgenerated on the secondary side of the transformer.

[0003] Such DC-DC converters are also known as a decisive assembly ofswitched-mode power supplies. They comprise a switched-mode power supplyinput circuit for connection to the electricity power grid and for thegeneration of an intermediate circuit DC voltage. The intermediatecircuit DC voltage feeds the incorporated DC-DC converter.

[0004] Numerous circuits for DC-DC converters are known. They comprise,on the one hand, very simple circuits in which the inverter onlyconsists of a single controlled switch and through which circuits theswitched AC voltage is directly fed to the primary side of thetransformer, so that the primary-side circuit thus consists of only theprimary side of the transformer. On the other hand, also resonantconverters are known in which a primary-side circuit is supplied with aswitched-mode AC voltage generated by a halfwave bridge or fullwavebridge, which primary-side circuit is built up from at least onecapacitance and at least one inductance, the inductance often not beingpresent as a discrete component, but as the stray inductance of thetransformer.

[0005] In EP 0 898 360 is described a method and a device forcontrolling a DC rectifier with an AC intermediate circuit. An inverterwith a controlled halfwave bridge or fullwave bridge generates an ACvoltage which is transformed via a transformer and rectified on theoutput side. The secondary side of the transformer used then comprises awinding with a middle tap while the output voltage is rectified bytwo-way rectification. In the publication the problem is discussed ofthe saturation of the magnetic flux in the transformer core. To achievea waveform here which shows the least possible saturation it is proposedthat the switched-mode AC voltage generated by the inverter issymmetrical, i.e. that within a predefined time interval a voltage pulseis generated that is as long positive as it is negative. Thispublication, however, exclusively discusses a control method and not aregulation of the output voltage. In this case a converter topology isdealt that which does not utilize a resonant arrangement.

[0006] In resonant converters it is a known fact that the output voltageis regulated, the drive of the inverter forming the setting variablethen. If, for example, a halfwave bridge generates a pulse-widthmodulated voltage for supplying power to a resonant arrangementmentioned above, the output voltage can be regulated by varying thefrequency of the pulse-width modulated voltage: the closer the switchingfrequency is to the resonant frequency, the more noticeable is aresonant voltage increase and the higher the (rectified) output voltageis.

[0007] U.S. Pat. No. 5,986,895 describes a DC current converter whichcomprises an inverter driven by a pulse-width modulation, which invertersupplies power to a resonant circuit in the form of a resonantcapacitance and the primary side of a transformer. The secondary-sidevoltage of the transformer is rectified and filtered at a secondarywinding with middle tap in two branches of one diode each to generate aDC output voltage. The output voltage is regulated via the drive of theinverter with variable pulse width. To reduce losses during switchingthe current through the resonant circuit is measured and the switchingtimes are selected such that—when a minimum and a maximum frequency areadhered to—switching only takes place when the current reaches a lowerthreshold value. U.S. Pat. No. 5,986,895 additionally describes inaddition the problem of losses during the rectification of asymmetricaloutput voltages. This problem is solved by the use of secondary-sidesynchronous rectifiers instead of diodes.

[0008] As a matter of fact secondary-side asymmetries in the pathscontaining rectifier elements (diodes) form a problem for DC-DCconverters. They show because with multipath rectification (for exampleby a four-diode bridge or by two single diodes with a winding with amiddle tap) the individual branches and thus the individual rectifierelements are loaded differently i.e. different powers, voltages orcurrents are to be managed. This leads to the fact that when the circuitis designed the asymmetrically loaded components are to be designedaccording to their respective maximum load. For example, components areto be dimensioned with a larger, for example double power dissipation,so that partially additional cooling measures are necessary. Also anyoutput filter for filtering the rectified output voltage is to be ableto process the lower frequency portions (subharmonics) resulting fromasymmetry and to have a larger structure for this and also for a highercurrent standing wave ratio.

[0009] A reason for such asymmetries may be for example tolerances inthe stray inductance of the output-side windings or tolerances in theforward voltages of the diodes. Building the circuits for avoiding suchasymmetries with the aid of components that have smaller tolerancesleads to strongly increased cost. Furthermore, the tolerances cannotalways be avoided because for example the forward voltage of a diodedepends on temperature.

[0010] Accordingly, it is an object of the invention to provide a DC-DCconverter and a regulation method for a DC-DC converter in whichconverter and method asymmetrical loads of the secondary-side rectifierare avoided to a maximum extent.

[0011] This object is achieved by a DC-DC converter as claimed in claim1, a regulation method as claimed in claim 12 and a switched-mode powersupply as claimed in claim 13. Dependent claims relate to advantageousembodiments of the invention.

[0012] According to the invention asymmetrical loads are avoided in thatan electrical magnitude is measured and a symmetry criterion is appliedthereto. Accordingly driving the inverter then provides that it isexcited so that the output load is symmetrical.

[0013] In the DC-DC converter according to the invention is provided ameasuring arrangement for an electrical magnitude where various primaryand secondary-side magnitudes are involved. For example, theprimary-side current can be measured by the transformer. Likewise it ispossible to measure the voltage at a primary-side capacitance. A furtherpossibility comprises the measuring of a secondary-side magnitude,preferably the rectified secondary-side voltage. Accordingly suitablemeasuring arrangements are sufficiently known to the person of ordinaryskill in the art.

[0014] A symmetry calculator unit uses the measuring result to calculatetherefrom a parameter for the symmetry deviation from the electricalmagnitude. Preferably the waveform of the electrical magnitude isconsidered here and for an observation interval the symmetry thereof isdetermined i.e. the deviation from a symmetrical waveform. Variouscriterions may be used for this according to which the symmetry isjudged and a parameter for the deviation of symmetry is formed.

[0015] According to a first further embodiment of the invention aparameter for the symmetry deviation is formed in which thesubstantially periodic waveform of the electrical magnitude underconsideration, preferably the voltage on a primary-side capacitance, isexpressed in a time interval with at least a local minimum and a localmaximum of the waveform. A parameter for the symmetry deviation hereforms a magnitude that depends on the difference between the values ofthe respective peak values. “Depends” is here to be understood to meanthat the difference between the values can directly express the valuefor the symmetry deviation, but it is also possible for furthermathematical operations to be applied to this value, for example,multiplication by a constant factor.

[0016] A further proposal is that a magnitude is given as a parameterfor the symmetry deviation, which magnitude is calculated from thedifferent-sized deviations of a maximum and a minimum value from a meanvalue of the electrical magnitude. The respective deviations(differences) are determined and compared with each other (difference ofthe values). The parameter for the symmetry deviation is calculated fromthis difference while here too further mathematical operations can beapplied.

[0017] According to a further proposal a criterion is used as aparameter for the symmetry deviation, according to which criterion thewaveform of the electrical magnitude is considered and within a firsttime interval an extreme i.e. a minimum or maximum value is determined.Likewise an extreme value is determined for a second time interval. Aparameter for the symmetry deviation can be calculated from thedifference between the extreme values. This parameter is particularlysuitable for considering a rectified magnitude, for example of thesecondary-side rectified voltage. Maximum values in successive timeintervals, preferably in the excitation intervals of the pulse widthmodulated switched AC voltage, are a parameter for the power transferredvia the respective branch of the rectifier. A deviation of the extremevalues of the two time intervals considered is thus a parameter forasymmetrical load.

[0018] It is generally possible for the symmetry calculator unit tofirst calculate an intermediate magnitude from the measured magnitude.For example, by integrating the current at a capacitance an intermediatevalue whose waveform corresponds to that of the voltage at thecapacitance can be determined at a capacitance over time. The criterionsindicated for determining the symmetry deviation can then be used forthis intermediate magnitude.

[0019] In addition, more criterions may be found which are suitable as aparameter for the symmetry deviation. A person of ordinary skill in theart will choose the most favorable criterion as regards cost andrequirements for a concrete application.

[0020] The symmetry deviation determined thus is applied according tothe invention to a symmetry regulation device which predefines the driveof the inverter so that the symmetry deviation is regulated to zero. Ithas turned out that an even load of the output rectifiers isachieved—better or worse depending on the criterion used—by regulatingthe electrical magnitude in a way that a symmetrical waveform isachieved relative to the respective criterion used. Even with componentsthat have relatively large tolerances it is possible to attain an evenload of the output rectifier.

[0021] In a preferred embodiment of the invention the inverter is drivenso that it produces a pulse width modulated AC voltage. This isunderstood to mean an AC voltage which is featured by a switchingfrequency and a duty cycle, while within each time interval whoseduration is determined by the switching frequency, first a positive andthen a negative voltage pulse is generated (fullwave bridge) or first apositive voltage pulse and then a zero output voltage is generated for aperiod of time (halfwave bridge). The duration of the positive voltagepulse is predefined by the duty cycle (ratio of the duration of thepositive voltage pulse to the total time of the interval) and theduration for which the voltage is zero or negative is predefined by therest duration of the interval. The symmetry regulation device herepreferably uses the duty cycle as a setting quantity to regulate thesymmetry deviation to zero i.e. to achieve a symmetrical waveform of theelectrical magnitude relative to the respective symmetry parameter.

[0022] A symmetry regulation device that respectively predefines theduty cycle can be combined in a simple manner with a conventional knownregulation device for regulating the output voltage. The outputregulation device can then regulate, for example, the output voltage andpredefine for this the frequency of the pulse width modulated AC voltagein known manner, whereas the symmetry regulation device predefines theduty cycle. In this way a DC-DC converter can be arranged for producinga regulated DC output voltage while the power semiconductors(rectifiers), the transformer and the output filters, as required, canbe utilized optimally.

[0023] A symmetry regulation device and a symmetry calculator unit areunderstood to mean purely functional units, i.e. in a concrete devicesuch units need not of necessity be separate assemblies. A symmetrycalculator unit as well as a symmetry regulation device can be realizedas an analog circuit. A person of ordinary skill in the art is familiarwith respective circuits for making mathematical calculations (forexample, difference formation, multiplication by a constant, averaging,integration etc.). Also analog circuits for realizing a regulationdevice are known such as a simple proportional regulator or a PIDregulator. Alternatively, the symmetry calculator unit and/or thesymmetry regulation device may also be realized as a discrete orintegrated digital circuit, or completely or in part designed as aprogram running on a microprocessor or signal processor.

[0024] In a concrete realization it is also possible to realize in oneassembly both the symmetry regulation device according to the inventionand a known output regulation device for the regulation of the outputvoltage.

[0025] These and other aspects of the invention are apparent from andwill be elucidated with reference to the embodiments describedhereinafter.

[0026] In the drawings:

[0027]FIG. 1 shows a circuit diagram of a first embodiment of a DC-DCconverter according to the invention;

[0028]FIG. 2 shows a diagram of the waveform of electrical magnitudes inFIG. 1;

[0029]FIG. 3 shows a second diagram of the waveform of electricalmagnitudes in FIG. 1;

[0030]FIG. 4 shows a block circuit diagram of a general symmetrycalculator unit and of a symmetry regulation unit;

[0031]FIG. 5 shows a schematic circuit diagram of a symmetry regulationunit;

[0032]FIG. 6 shows a schematic circuit diagram of a symmetry regulationunit;

[0033]FIG. 7 shows a circuit diagram of a part of a fourth embodiment ofa DC-DC converter according to the invention;

[0034]FIG. 8 shows a schematic circuit diagram of a symmetry calculatorunit and of a symmetry regulation device;

[0035]FIG. 9 shows a diagram of the waveform of electrical magnitudes ofthe circuit shown in FIG. 7;

[0036]FIG. 10 shows a circuit diagram of a part of a fifth embodiment ofa DC-DC converter according to the invention.

[0037]FIG. 1 shows a partially schematic circuit diagram of a DC-DCconverter 10. It comprises an inverter 12 supplied with power by a DCvoltage source Vdc, which inverter is arranged here as an asymmetricallyswitching halfwave bridge. The switched AC voltage vHB generated by theinverter 12 feeds a resonant arrangement 14, which includes in series aresonant capacitance C, an inductance L and the primary side of atransformer T. The inductance L can in addition to being represented bya discrete component, also be represented by the primary-side strayinductance of the transformer T.

[0038] On the secondary side the transformer T has two secondarywindings Lsa, Lsb which are arranged as one winding with a middle tap.The two power semiconductors (diodes) Da, Db are connected to the endsof the secondary winding, so that altogether a two-way rectification iseffected on the secondary side with two rectifier elements. Therectified voltage is filtered in a filter F to an output voltage Vo towhich a load 20 is connected. Embodiments of filter circuits F have longsince been known to the expert. Examples comprise capacitive, inductiveor combined output filters F.

[0039] The inverter 12 has two controlled switches, normally arranged asfield effect transistors. They are driven by a halfwave bridge driver Dto which is applied a control signal (pulse signal) by a modulator M.Driver D and modulator M are part of a measuring and regulatorarrangement 16. The control signal then directly predefines the positionof the switches of the halfwave bridge of the inverter 12; with a Highlevel the upper switch is closed and the lower switch is open and with aLow level the lower switch is closed and the upper switch is open.Depending on the position of the switches there is a voltage vHB of +Vdcor zero present on the resonant circuit 14.

[0040] The drive by the modulator M and the driver D is effected in theway that the output voltage of the inverter 12 is a pulse widthmodulated voltage. The waveform of the pulse width modulated voltage vHBis shown at the top in FIG. 2. In a time interval of duration T_(ges)the pulse width modulated voltage has a positive voltage pulse ofduration T_(A) and a waveform of duration T_(B), during which there is azero voltage. The waveform of such a pulse width modulated voltage canbe unambiguously determined by stating the switching frequencyf=1/T_(ges) and the duty cycle δ=T_(A)/T_(ges). This is of course anidealized waveform. In practice the generated pulses—on the basis offinite edge steepness—rather have a trapezoidal form. To avoidshort-circuits, dead times are additionally used between the pulses.

[0041] The output voltage Vo is regulated in a manner known per se whilean increase of resonance is used. For this purpose the regulation device16 comprises an output regulator 18 (PID regulator). A measuring devicemeasures the output voltage Vo at the load terminals and supplies sameto the output regulator 18. Furthermore, the set value of the outputvoltage Vo is applied to the output regulator 18 (not shown). The outputregulator 18 predefines via the signal Sfrq the switching frequency ofthe switches of the inverter 12 and thus the pulse width modulatedvoltage to the modulator M. Inverter 12 is then always operated at aswitching frequency that is clearly above the known resonantfrequency/frequencies of the resonant arrangement 14. By reducing thefrequency i.e. a respective signal Sfrq, the output voltage Vo canconsequently be increased and vice versa. In this way the outputregulator 18 produces the setting signal Sfrq by which the voltage Vo isregulated to the set value.

[0042] The regulation device 16 further includes a unit 22 whichcomprises a symmetry calculator unit and a symmetry regulator. Such aunit 22 is represented in general in FIG. 4 in the form of a blockcircuit diagram.

[0043] As shown in FIG. 4, the unit 22 comprising a symmetry calculatorunit EC and a symmetry regulator R takes up a measuring signal Smcs thathas an electrical magnitude. The symmetry calculator unit EC determinesfrom the measuring signal Smes a parameter for the deviation of thewaveform of Smes from a waveform that is symmetrical in accordance witha symmetry criterion. To obtain as symmetrical a waveform as possible,this parameter is minimized i.e. the parameter determined for thedeviation Serr is treated as a regulation error to be cancelled andapplied to the symmetry regulator R. Via the duty cycle which ispredefined as signal Sduty by the symmetry regulator R, Smes isregulated in such a way that it satisfies in the best way possible thesymmetry criterion used by the symmetry calculator unit EC and Serr isminimized.

[0044] Depending on the arrangement, various electrical magnitudes ofthe circuit are considered which may be measured and applied as ameasuring signal Smes to the unit 22. This includes primary-sidemagnitudes with which a measuring unit 24 as shown in FIG. 1 capturesthe primary-side current iC or the voltage vC on the primary-sidecapacitance C and applies same as a measuring magnitude Smes to the unit22. As explained hereafter, it is also possible that secondary-sidemagnitudes are considered.

[0045] A symmetry criterion is used for the primary-side measuringmagnitudes, which criterion looks at the periodic waveform of themagnitude and derives from this a parameter for the symmetry by acomparison of maximums and minimums. In general the symmetry deviationSerr determined by this criterion can be written as:

-i Serr=const*[max{Smes}−av{Smes}+min{Smes}−av{Smes}]

[0046] where

[0047] Smes is the measured electrical magnitude,

[0048] const is a measuring or gain constant,

[0049] the operations max( ) and min( ) produce the respective maximumor minimum of the magnitude Smes considered within the respectiveinterval. (At least one period of the signal should be used here as ameasuring interval. Since a change of the waveform from one period toanother, however, may be considered small, maximum and minimum can alsobe determined over a number of successive periods).

[0050] the operation av( ) produces the temporal mean value i.e. the DCcomponent of the magnitude under consideration.

[0051]FIG. 5 shows the respective structure of the symmetry calculatorunit EC. The measuring signal Smes is first subjected to a preprocessingoperation in general represented by the box 50. This is an input filterwhich can for example remove the DC component from Smes. Depending onthe arrangement, the filter 50 may, however, also be omitted or alsoexecute other operations. The filtered signal is applied to a maximumunit 26 and a minimum unit 28 which determine the maximum and minimum ofthe value, respectively. The symbolic representation of FIG. 5 refers toan analog circuit having this function. Obviously, digital arrangementsare also conceivable.

[0052] The values of maximum and minimum are subtracted (since it isassumed in FIG. 5 that the minimum has a negative value, a simpleaddition will do). After multiplication by the measuring constant constan error signal Serr arises which represents a parameter for thesymmetry deviation.

[0053]FIG. 6 shows the structure of the regulator R to which thesymmetry deviation Serr is applied as an error parameter to beregulated. For the predefinition of the duty cycle controlled by Sduty,the regulator R uses a predefined value δ₀ of for example 50%. Aregulator G_(R), for example a PID, PI or only I-regulator adds a(positive or negative) deviation from the basic setting δ₀ in dependenceon Serr. Based on the I-portion of the regulator G_(R) anon-disappearing contribution is added to δ₀ even with disappearingSerr, so that the accuracy of the preset value δ₀ does not have anyinfluence on the result of the regulation.

[0054] The signal Sduty predefined by the symmetry regulator R withinthe unit 22 is applied to the modulator M which in dependence hereonapplies a pulse signal to the driver D to drive the halfwave bridge 12while the duty cycle of the thus generated pulse width modulated voltagevHB corresponds to the predefined value Sduty.

[0055]FIG. 2 shows the waveform of various electrical magnitudes of theDC-DC converter of FIG. 1. It comprises not only the pulse widthmodulated voltage vHB and the primary-side current iC, but also thevoltage vC on the capacitance C as well as the secondary-side currentsia, ib via the rectifier diodes Da, Db. FIG. 2 shows the case of anasymmetrical load. As shown in FIG. 2 a clearly smaller power ia(integrally via the current) flows across the first rectifier element Dain each time interval T_(ges) and a clearly larger power ib across thesecond rectifier element Db. This is the asymmetrical load mentionedabove which, with the design of for example the diodes Da, Db mentionedabove, leads to the fact that with respect to their relatively highpower loss (cooling etc.) they have to have a considerably higher valuethan would actually be necessary. For with the design always the worstcase scenario of the (asymmetrically) higher load for each of the diodesis to be started from.

[0056] Hereafter will be explained three examples for measuredprimary-side magnitudes and application of the general symmetrycriterion for these magnitudes described above.

[0057] A first proposal for a symmetry criterion relates to the waveformof the voltage vC on the primary-side capacitance C. The waveform of therespective voltage vC is approximately sinusoidal as is shown in FIG. 2.As explained above there is a comparison of the maximums and minimums ofthe temporal waveform.

[0058] The respective magnitudes are individually shown in FIG. 3. Thevoltage vC varies between the maximum value vCmax and the minimum valuevCmin around its temporal mean value vCO. The deviation of the “height”of the maximum above the temporal mean value or of the minimum belowthis mean value, respectively, is substantial to the symmetrical load ofthe total circuit. The respective magnitudes are shown in FIG. 3 as dvC1and dvC2. As a measure for the symmetry deviation is used the differenceof the values from dvC1 and dvC2.

Serr=const*((vCmax−vC 0)+(vCmin−vCo)).

[0059] The respective first embodiment of a DC-DC converter of FIG. 1comprises a measuring arrangement 24 which detects the voltage vC on thecapacitance C. The symmetry calculator unit EC of the first embodimentis arranged in accordance with FIG. 5 and includes in the filter SO aunit for forming and subtracting the temporal mean value of vC.Corresponding circuits or corresponding algorithms for this,respectively, are widely known. In this way the above expression issimplified because only the AC component ac(vC) is present, so that thesubtraction of vCo can be omitted.

[0060] The symmetry calculator unit EC further comprises a circuit or analgorithm, respectively, to evaluate the above expression and respectivecalculation of Serr. The regulation by the respective regulator G_(R) ismade with the predefined value that with a value Serr>0 the duty cycleis reduced and with Serr<0 the duty cycle is increased.

[0061] In a second embodiment the waveform is shown of the primary-sidecurrent iC which is measured by a current sensor 24 and applied to thecalculator unit 22.

[0062] The second proposal for the symmetry criterion used by thesymmetry calculator unit EC will be explained with reference to thewaveform for the current iC, which waveform is shown in FIG. 2 at thebottom. As can be seen there, the waveform of the primary-side currentiC is periodic with the excitation frequency. It is an undulatingwaveform with minimums and maximums, but no pure sinusoidal oscillation.To form a measure for the symmetry deviation, the minimum and maximumvalue of the waveform of iC is determined in the excitation intervalT_(ges). As a parameter for the symmetry deviation is used thedifference of the absolute values of the maximum and minimum values. Asubtraction of the mean values (DC component) may be omitted because thecurrent through a capacitance has no DC component.

[0063] Thus the symmetry deviation is calculated by

Serr=const*(Imax+Imin).

[0064] The symmetry deviation is settled in that with Serr>0 the dutycycle is increased and with Serr<0 the duty cycle is reduced.

[0065] Structure and way of operation of the unit 22 of the secondembodiment correspond to FIG. 5. The symmetry calculator unit ECprocesses measuring values iCmes of the current sensor 24 for theprimary current iC. The second embodiment of a symmetry calculator unitEC uses a somewhat simplified symmetry criterion for the waveform of themagnitude under consideration, here iCmes via the comparison of thevalues of minimum and maximum without a DC component. Accordingly, alsothe conversion from a point of view of circuitry (for example as ananalog circuit) or from a point of view of programming (when aregulation algorithm is used) is a little less expensive. Experimentshave shown that a primary-side current iC symmetrically regulated withrespect to this criterion leads to an improved symmetrical load of theoutputs, here particularly the output diodes Da, Db. Yet the firstproposal is preferred (taking the voltage vC into consideration), sinceeven better results were achieved here. Also the measurement is oftensimpler because in general voltages can be measured at less cost thancurrents. For a concrete application the person of ordinary skill in theart will have to decide based on requirements which embodiment is used.Taking the current iC into consideration is advantageous, for example,if this current is measured for other purposes anyway.

[0066] In accordance with a third proposal the general symmetrycriterion is used for an intermediate magnitude calculated from theprimary-side current iC. First of all a signal int(iC) is formed as anintermediate magnitude by temporal integration of the current iC, whichsignal int(iC) as regards its waveform corresponds to vC. Represented asa formula, Serr is calculated as follows:

Serr=const*[max{int(iC)}−av{int(iC)}+min{int(iC)}−av{int(iC)}]

[0067] where

[0068] int(iC) is the temporal integral of the measuring magnitude iC,

[0069] max ( ) is the maximum in an interval under consideration,

[0070] av ( ) is the temporal mean value of the magnitude underconsideration, here iC. Also this expression can be simplified byremoving the DC component of the magnitude iC under consideration, asexplained above.

[0071] Accordingly, also in a third embodiment the DC-DC converter hasthe structure of FIG. 1. However, the unit 22 here includes a symmetrycalculator unit EC (not shown) with respective units for evaluating theabove expression which are known to the person of ordinary skill in theart. The formation of intermediate magnitude int(iC) by integration canbe effected by the input filter 50. The regulation by the downstreamregulator R is effected with the predefined value that for Serr>0 theduty cycle is reduced and for Serr<0 the duty cycle is increased.

[0072] In the first to the third embodiment explained above,primary-side magnitudes were measured and the general symmetry criterionwas applied to them. However, also secondary-side magnitudes may beconsidered. With the circuit according to FIG. 1 the waveform of thesecondary-side magnitudes is substantially determined by therectification. Therefore, another symmetry criterion is applied herewhich comprises the comparison of the height of the successive maximums.Depending on the respective use and magnitude considered, additionaloperations may be applied, as is shown in the following examples.

[0073]FIG. 7 shows a part of a fourth embodiment of a DC-DC converter.FIG. 7 only shows the output filter F; the rest of the DC-DC converterhas the same structure as that shown in FIG. 1. This is the reason whythe structure and the function of the corresponding parts are notfurther discussed here.

[0074] The filter F is arranged as a Pi filter, comprising a capacitanceC1F, inductance LF and a further capacitance C2F. A current iC1F flowsvia the capacitance C1F. The input voltage of the filter is referred toas Vab, the output voltage as Vo.

[0075] The waveforms of the electrical magnitudes of FIG. 7 are shown inFIG. 9. The current iab corresponds in essence to the current iC1F as aresult of the filter operation of LF. The voltage Vab shows aconsiderable undulation because the capacitor has non-disappearingvalues for series inductance and series resistance (not shown).

[0076] In FIG. 9 the waveform of iab shows an asymmetrical load of theoutput rectifier Da, Db. Successive maximums alternately have differentvalues, which in the two-way two-diode rectification shows theasymmetrical spreading of power over the two rectifier elements Da, Db.

[0077] In FIG. 9 is shown a voltage Sdrv which as a switching signalcorresponds to the pulse signal generated by the modulator M, as isshown in FIG. 7. The voltage Sdrv is used as a switching signal todistinguish the time interval TA from the time interval TB (excitationinterval of the pulse width modulated voltage vHB). Within this intervalTA, TB the waveform of the voltage Vab is considered to obtain therefroma parameter for the symmetry deviation.

[0078] The structure of the respective unit 22 is shown in FIG. 8. Thevoltage Vab measured by a voltage measuring arrangement 60 is applied tothe unit 22 and there filtered by an RC low-pass filter 72 to obtain thevoltage vabf. Depending on the status of the signal Sdrv a switchingunit 74 applies the voltage vabf to a first sample and hold circuit S&Haor to a second sample and hold circuit S&Hb. The first sample and holdunit S&Ha is allocated to the first excitation interval TA and thesecond sample & hold unit S&Hb to the second excitation interval TB. Apeak value is determined by means of the sample and hold units S&Ha,S&Hb within the respective considered intervals. The downstreamcomparator 76 generates an error signal Serr which is applied to theregulator R. The regulator R regulates the error signal Serr bypredefining the duty cycle by means of the signal Sduty, where itincreases the duty cycle when Serr>0 and reduces the duty cycle whenserr<0.

[0079] The relevant magnitudes in this method for determining a symmetrydeviation are shown at the bottom in FIG. 9. The low-pass filteredvoltage vabf has a maximum within the first excitation interval TA whichis featured by the lower dotted line. The waveform of vabf shows ahigher maximum within the second excitation interval TB, which highermaximum is featured by the upper dotted line. The symmetry deviation isgiven by the signal Serr indicated by the two-way arrow between thedotted lines.

[0080] A fifth embodiment of a DC-DC converter (not shown) utilizes thesecondary current iab. The heights of successive maximums is comparedfor respective successive excitation intervals TA, TB. For this purposeis used a unit 22 as shown in FIG. 8, from which, however, the inputfilter 72 is omitted.

[0081]FIG. 10 shows a part of a sixth embodiment of a DC-DC converter.Only the transformer T is shown here with a secondary-side circuit. Forthe rest the structure corresponds to that of FIG. 1.

[0082] In FIG. 10 the transformer T comprises only a secondary winding.Here a rectifier 90, for example a commercially available bridgerectifier, is used. Rectification is effected with the aid of a fullwavebridge with four rectifier elements Da1, Da2, Db1, Db2. Accordingly therectified voltage Vab shows the typical waveform of a voltage rectifiedby a full-bridge. Also in the sixth embodiment an output filter F isarranged upstream of the load 20.

[0083] Also for the sixth embodiment of the DC-DC converter are arrangeda measuring unit for detecting an electrical magnitude, a symmetrycalculator unit and a symmetry regulation unit (none of them shown).Here the same magnitudes and symmetry criterions are involved as in theembodiments shown above to obtain a symmetrical load of the rectifierelements Da1, Da2, Db1, Db2. In the sixth embodiment comprising onebridge rectifier the secondary-side magnitudes ia, ib, iab, vab inessence show a waveform as shown in FIG. 9. A difference is thatdifferent output-side inductances are omitted here as an error sourcefor asymmetry. Disturbances of symmetry as a result of different diodevoltages may double in the worst case, however.

[0084] To effect the regulation in accordance with the invention withthe object of a symmetrical load also secondary-side non-rectifiedmagnitudes can be measured in addition to the measuring magnitudes andsymmetry criterions explained above with reference to the otherembodiments. The symmetry criterions indicated above with reference tothe first to third embodiments (comparison of maximum and minimum) canbe used.

[0085] In FIG. 1 the DC voltage supply Vdc is symbolically shown as avoltage source. In a switched-mode power supply (not shown) Vdc is theintermediate circuit voltage of the switched-mode power supply whichvoltage is delivered by a power supply input circuit. A power supplyinput circuit is used for connection to an AC voltage supply, forexample the electricity power grid. To generate an intermediate circuitDC voltage Vdc the input AC voltage is rectified and filtered dependingon requirements in the manner known to a man of ordinary skill in theart.

[0086] The invention can in this respect be summarized in that a DC-DCconverter, a regulation method for a DC-DC converter and a switched-modepower supply are proposed. The DC-DC converter comprises an inverter anda primary-side circuit with a transformer whose secondary-side voltageis rectified to generate an output DC voltage with at least onerectifier. To avoid an asymmetrical load which particularly exhibitsitself in a different load of the rectifier elements (powersemiconductors), an electrical magnitude of the DC-DC converter ismeasured. This magnitude may be, for example, the primary-side current,a primary voltage on a capacitance or a secondary-side, rectifiedvoltage. From the measurement of the magnitude is obtained a parameterfor the symmetry deviation for which various methods of symmetrymeasurements are proposed. A symmetry regulation unit utilizes the driveof the inverter, for example, the duty cycle, the pulse-width modulatedvoltage delivered by the inverter to minimize the value of the symmetrydeviation. This achieves an even spreading of the power over thesecondary-side rectifier elements.

1. A DC-DC converter comprising an inverter (12) for generating aswitched AC voltage, by which a primary-side circuit (14) is suppliedwith power, which comprises the primary side of a transformer (T), onthe secondary side of the transformer (T) being provided at least onerectifier (Da, Db, i90) for generating at least an output DC voltage(Vo), a measuring unit (24) for measuring an electrical magnitude (iC)of the circuit (24), and a symmetry calculation unit (EC) whichcalculates from the measurement a parameter for the symmetry deviationof the electrical magnitude (iC, VC, Vab, iab), a symmetry regulationunit (R) being provided to change the drive of the inverter (12) independence on the symmetry deviation so that the symmetry deviation isminimized.
 2. A DC-DC converter as claimed in claim 1, in which theinverter (12) is driven so that it supplies a pulse width modulatedvoltage, and the symmetry regulation device (22) is arranged so that itregulates as a setting variable the symmetry deviation to zero via theduty cycle of the pulse width modulated AC voltage.
 3. A DC-DC converteras claimed in one of the preceding claims, in which the primary-sidecircuit is a resonance arrangement (14) comprising at least onecapacitance C.
 4. A DC-DC converter as claimed in one of the precedingclaims, in which the measuring arrangement (24) is designed such thatthe primary-side current (iC) is measured by the transformer (T).
 5. ADC-DC converter as claimed in one of the preceding claims, in which themeasuring arrangement (24) is designed so that the voltage (vC) ismeasured at a primary-side capacitance (C).
 6. A DC-DC converter asclaimed in one of the preceding claims, in which the measuringarrangement (60) is designed so that a rectified secondary-sideelectrical magnitude (Vab, iab) is measured.
 7. A DC-DC converter asclaimed in one of the preceding claims, in which an intermediatemagnitude (VC) is calculated from a measured electrical magnitude (iC).8. A DC-DC converter as claimed in one of the preceding claims, in whichthe symmetry calculation unit (EC) is arranged so that a magnitude iscalculated as a parameter for the symmetry deviation, which magnitudedepends on the difference between the negative and positive peak valueof at least one of the electrical magnitudes (iC, VC).
 9. A DC-DCconverter as claimed in one of the preceding claims, in which thesymmetry calculation unit (EC) is arranged so that a magnitude isproduced as a parameter for the symmetry deviation which magnitude iscalculated from the difference between the values of the deviation of amaximum of at least one of the electrical magnitudes (iC, vC) from amean value of this electrical magnitude and the deviation of a minimumof at least one of the electrical magnitudes (iC, vC) from the meanvalue of this magnitude.
 10. A DC-DC converter as claimed in one of thepreceding claims, in which the symmetry measuring arrangement (EC) isdesigned so that a magnitude is produced as a parameter for the symmetrydeviation which magnitude is calculated in accordance with the followingexpression: const*(max(ac(Smes))+min(ac(Smes)))  where const is aconstant factor max( ) is a function for determining a temporal maximumvalue, min( ) is a function for determining a temporal minimum value,ac( ) is a function for determining a signal value without a DCcomponent and Smes is a measured electrical magnitude
 11. A DC-DCconverter as claimed in one of the preceding claims, in which thesymmetry measuring unit (EC) is arranged so that a magnitude is producedas a parameter for the symmetry deviation which magnitude is calculatedfrom the difference between the values of a first extreme of anelectrical magnitude (iab, Vab) within a first time interval (TA), andof a second extreme of the electrical magnitude within a second timeinterval (TB).
 12. A regulation method for a DC-DC converter (10) theDC-DC converter (10) comprising an inverter (12) for generating aswitched AC voltage and a primary-side circuit (14) supplied with thisvoltage, the circuit comprising the primary side of a transformer (t),at least one rectifier (Da, Db) for the generation of at least oneoutput DC voltage (Vo) being arranged on the secondary side of thetransformer (T), an electrical magnitude (iC, vC, iab, Vab) of the DC-DCconverter being regulated such that a parameter for the symmetrydeviation of the electrical magnitude (iC, vC, iab, Vab) is minimized,and the drive of the inverter (12) being used as a setting variable. 13.A switched-mode power supply comprising a power supply input circuit forconnection to a voltage supply and for the generation of an intermediatecircuit DC voltage (Vdc), and a DC-DC converter (10) as claimed in oneof the claims 1 to 11 with the intermediate circuit DC voltage (Vdc).